1. Field of the Invention
The invention relates to connecting rods, and more particularly to a variable length connecting rod for varying compression ratio and associated methods and apparatus.
2. Background Art
Changing the compression ratio of an internal combustion engine by variation of connecting rod length has been the subject of a number of proposals and patents. See, e.g., U.S. Pat. No. 4,834,031. However, design of a commercially viable variable compression ratio (VCR) engine has proven to be an elusive goal, due largely to the difficulties associated with providing a suitable mechanism for changing the connecting rod length.
U.S. Pat. No. 4,834,031 discloses varying compression ratio by rotating an eccentric bearing on the pin that joins the connecting rod to the piston, which is referred to therein as the piston pin. The bearing has an eccentric bore into which the piston pin is fitted. A lock pin is employed to lock the eccentric bearing against rotation relative to the connecting rod for operation in high compression ratio mode. Oil pressure is employed to unlock the eccentric bearing by shifting the lock pin out of engagement with the eccentric bearing for operation in low compression ratio mode. Oil is provided for control purposes through passages in the crankshaft and the connecting rod.
In the mechanism of U.S. Pat. No. 4,834,031, there is a need to shift the lock pin rapidly and precisely within a narrow time interval. To achieve sufficiently rapid shifting, inertia and friction between the components must be overcome, typically by fluid pressure, but the magnitude of fluid pressure available for this purpose may be limited.
In the mechanism described in the above referenced '031 patent, shifting of the lock pin must occur within a very narrow window of time during which the eccentric bearing and connecting rod are aligned. Implementation of this mechanism in a commercially viable engine would be difficult in light of the need to overcome friction and inertia to accelerate the pin sufficiently rapidly to insert it into and withdraw it from the eccentric bearing within the available window of time. Friction associated with sliding the pin into and out of the eccentric bearing would be particularly problematic if the openings for the lock pin in the eccentric bearing and connecting rod were not precisely aligned at the critical point in time.
While the oil pressure applied to the lock pin may theoretically be raised to a high enough level to overcome the friction and inertia, in practice oil leakage may become prohibitive at high pressures in systems of this type where oil is transmitted between components that are moving relative to one another. Providing oil at a high enough pressure to overcome the friction and inertia in the mechanism described in the '031 patent would be particularly difficult due to the need to transmit the oil along the length of the connecting rod to reach the piston pin.